If your defrost and floor vents are working your heater system is in safe mode. You probably have a vacuum leak. I have the same problem but I havent had time to check into it.
__________________
05 3500 Yellow 6 speed,Mega cab dually fenders,Rumble Bee Scoop,Edge Juice w/A2,Edge EZ,Smarty,CFM+,ATS Exhaust Mani,Silver 64,90's,Airdog 150,Valair 3600 DD,5" Silverline exhaust,8" tip,Icebox with torque tube,Quad Rail Gauge,Mag-Hytecs,Lorenz Level, 22.5's